Unit 4 ppt mt1

UNIT IV

SHEET METAL PROCESSES

Sheet metal characteristics - Typical shearing operations, bending and drawing operations – Stretch forming operations –– Formability of sheet metal – Test methods– Working principle and application of special forming processes - Hydro forming –

Rubber pad forming – Metal spinning – Introduction to Explosive forming, Magnetic pulse forming, Peen forming, Super plastic forming.

1IFETCE/MECH/MERGED/II YR/III SEM/ME2201/MTI/PPT/UNITIV/VER 1.2

• Introduction

• Shearing

• Sheet Metal Characteristics

• Test Methods for Formability of sheet metals

• Bending sheet and Plate

• Common Bending Operations

• Tube Bending and Forming

• Stretch Forming

• Deep Drawing

• Rubber Forming• Spinning• Super Plastic Forming• Explosive , Magnetic- Pulse, peen,

and Other Forking Processes• Manufacturing of Honeycomb

Structures

Sheet metal characteristics

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IFETCE/MECH/MERGED/II YR/III SEM/ME2201/MTI/PPT/UNITIV/VER 1.2

Shearing :• Sheet metal subjected to shear stress developed between

a punch and a die is called shearing

Schematic illustration of shearing with a punch and die, indicating some of the process variables. Characteristic features of (b) a punched hole and (c) the slug. Note that the scales of the two figures are different.

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Shearing • Shearing usually starts with formation of cracks on both the

top and bottom edges of the work piece.These cracks meet each other and separation occurs

Process parameters :• Shape of the material for the punch and die• Speed of the punching ,lubrication and clearance


Typical shearing operations• Several operations based on shearing performed Punching – sheared slug discardedBlanking – Slug is the part and the rest is scrap


• Die cutting

• Fine blanking

• Slitting

• Steel rules

• Nibbling

• Scrap in shearing

• Tailor welded blanks

TYPICAL SHEARING OPERATION


Shearing dies (OPERATIONS):

Clearance : function of type of material,its temper and its thickness and of the size of the blank and its proximity to the edges

• Clearance of soft materials are less than harder grades

Punch & die shapes :• Surfaces of punch and die are flat• Punch force builds rapidly and entire thickness is

sheared at same time.


Shearing dies

• Fig 16.3 (a) Effect of the clearance,c, between punch and die in the deformation zone in shearing .As the clearance increases the material trends to be pulled into the die rather than be sheared .

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Compound dies :• Several operations on the same strip performed in one

stroke at one station with a compound dies

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Progressive dies :• Parts produced with multiple operations such as, punching, blanking

and notching are made at high production rates in progressive dies.

16.11 (c) Schematic illustration of making a washer in a progressive die (d) Forming of the top piece of an aerosol spray can in a progressive die.

Note the part is attached to the strip until the last operation is completed10


Transfer dies :• Sheet metal undergoes different operations at different

stations in a straight line or circular path.


Other sheet metal cutting methods

• Band saw• Flame cutting• Laser beam cutting• Friction sawing• Water-jet cutting


Sheet metal characteristicsTYPICAL RANGE OF AVERAGE NORMAL ANISTROPY(Ravg) FOR

VARIOUS SHEET METALS

Table 16.2

Zinc 0.2

Hot rolled steel 0.8-1.0

Cold rolled rimmed steel 1.0-1.35

Cold rimmed aluminum–killed steel 1.35-1.8

Aluminum 0.6-0.8

Copper and Brass 0.8-1.0

Titanium 4-6


Bending and drawing operations

• Low carbon steels exhibit this behavior• This produces lueder’s bands(stretch strain marks)• These marks can be eliminated by reducing thickness of sheet 0.5 % to 1.5

% by cold rolling process

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Anisotropy :• Anisotropy is caused by thermo-mechanical processing

of sheet.2-types • Crystallographic anisotropy• Mechanical fibering

Strains on a tensile-test specimen removed from a piece of sheet metal.These strains are used in determining the normal and planar anisotropy of sheet metal

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Grain size :

• Grain size effects mechanical properties & surface appearance• The coarser the grain the rougher the appearance

Sheet metal formability :• Sheet metal undergoes two forms of deformation• Stretching• Drawing


Cupping test :

• The sheet metal specimen is clamped between two circular flat dies and a steel or round punch is pushed hydraulically into the sheet metal until a crack begins to appear on the stretched specimen

(a) A cupping test (the Erichsen test) to determine the formability of sheet metals. (b) Bulge-test results on steel sheets of various widths.The specimen farthest left is subjected to,basically,simple tension.The specimen farthest right is subjected to equal biaxial stretching

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BENDING OPERATIONS :• In bending outer fibers are in tension and inner fibers are in compression

(a) and (b) The effect of elongated inclusions (stringers) on cracking,as a function of the direction of bending with respect to the original rolling direction of the sheet. (c) Cracks on the outer surface of an aluminum strip bent to an angle of 90 degree.Note the narrowing of the top surface due to the Poisson effect

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Bend allowance :• Lb = alf ( R + KT)

Alf – bend angle (radians)

T-sheet thickness

R-bend radius

K-constant• Bend allowance for ideal case the sheet thickness ;k=0.5

Lb = alf(R+(T/2))

In practice the value of K ranges from 0.33-0.5• Minimum bend radius• Engineering strain on a sheet during bending

E = 1 / (2R/T)+1


Spring back :• In Bending ,after plastic deformation there is an elastic recovery

this recovery is called spring back.

• Spring back can be calculated approximately in terms if radii Ri and Rf

• Ri/Rf = 4 ( Ri Y / ET )3 – 3 (Ri Y /ET) + 1

Spring back Increases as (R/T ratio & yield stress of material ) increases as elastic modulus E decreases

Spring back in bending .The part tends to recover elastically after ending,and its bend radius becomes larger.Under certain conditions,it is possible for the final bend angle to be smaller than the original angle(negative spring)

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Cont’dCompensation for spring back• Over bending of part • Bottoming the punch – coin the bend area by

subjecting it to high localized compressive between the technique tip of the punch and the die surface.

• Stretch bending – Part is subjected to tension while being bent.In order to reduce spring back bending may also be carried to reduce spring back bending may also be carried out at elevated temperatures


Bending force :Maximum bending force, P = KYLT2

W

K – constant ranges from 0.3(wiping die) – 0.7(u-die)-1.3(V-die)Y – yield stressL- length of the bendT- thickness of sheet

For a V-dieMax bending force, P = (UTS)LT 2

W


Press brake forming – Used for sheets 7M(20ft) or longer and other narrow pieces

– Long dies in a mechanical or hydraulic press for small production runs

– Die material range from hardwood to carbides.


Roll bending :– Plates are bent using a set if rolls,various curvatures can

be obtained by adjusting the distance between three rolls

Common die-bending operations, Showing the die-opening dimensions, W, used in calculating bending forces

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Beading :

• The periphery if the sheet metal is bent into the cavity of a die

Bead forming with a single die (b) Bead forming with two dies,in a press brake

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Dimpling :

• First hole is punched and expanded into a flange• Flanges can be produced by piercing with shaped punch• When bend angle < 90 degrees as in fitting conical ends its called flanging


Hemming :• The edge of the sheet is folded over itself• This increases stiffness of the part

The metal strip is bent in stages by passing it through a series of rolls

Seaming :• Joining two edges of sheet metal by hemming specifically

shaped rollers used for watertight and airtight joints

Stretch forming operations


• Roll forming is used for continuous lengths of sheet metal

• Used for large production runs

Fig: Schematic illustration of the roll-forming process

Roll forming :

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Tube bending and forming

• Special tooling required to avoid buckling and folding

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Bulging• Process involves placing tabular,conical or curvilinear part into a split-

female die and expanding it


Formability of sheet metal

• Individuals are placed inside the parts and mechanically expanded in radial direction and finally retracted.

Stretch forming• Sheet metal clamped along its edges and stretched over

a die or form block in required directions.


Stretch forming

Schematic illustration of a stretch forming process. Aluminum skins for aircraft can be made by this process

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Test methods

• Punch forces a flat sheet metal into a deep die cavity

• Round sheet metal block is placed over a circular die opening and held in a place with blank holder & punch forces down into the die cavity

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Deep drawing process :• Wrinkling occurs at the edges


Deep drawability :

• Deep drawability is expressed in LDR

• Limiting drawing ratio (LDR)

LDR – Max blank dia/punch dia =Do/Dp

• Drawability of metal is determined by normal anisotropy( R ) or plastic anisotropy.

R = width strain / thickness strain =Ew /Et


Working principle and application of special forming processes

• Edges of cups may be wavy this phenomenon is called Earing

• The above condition is called planar anisotropy

• Del R = R0 – 2 R45 + R 90 / 2

Where Del R = 0 => no ears formed

Height of the ears increases Del R

increases. Earning in a drawn steel cup, caused by the planar anisotropy of the sheet metal

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Deep drawing Practice :• Blank holder pressure – 0.7% -1.0 % of Yield strength + UTS

• Clearance usually – 7% -14 % > sheet thickness

• Draw beads are used to control flow of blank into die cavity.

• Ironing is a process in which the thickness of a drawn cup is made constant by pushing of the cup through ironing rings.

• Redrawing – Containers or shells which are too difficult to draw in one operation undergo redrawing


• Drawing without blank holder :

Deep drawing without blank holder must be provided with sheet metal which is sufficiently thick to prevent wrinkling

Range : Do – Dp < 5T

Hydro forming


• Lowers forces and increases drawability

• commonly used lubricants are mineral oils ,soap solutions,heavy duty emulsions.

Tooling & equipment for drawing :• Tool & die materials are tool steels cast irons carbides • Equipment is hydraulic press or mechanical press

Lubricants


Rubber forming :• In bending and embossing of sheet metal , the female

die is replaced with rubber pad

Hydro-form (or) fluid forming process :• The pressure over rubber membrane is controlled

through out the forming cycle ,with max pressure up to 100 Mpi

• As a result the friction at the punch-cup interface increases ,this increase reduces the longitudinal tensile stresses in the cup and delays fracture


Rubber pad forming

• Shaping thin sheets by pressing them against a form with a blunt tool to force the material into a desired form

• Conventional spinning :

A circular blank if flat or performed sheet metal hold against a mandrel and rotated ,while a rigid metal is held against a mandrel and rotated ,wile a rigid tool deforms and shapes the material over the mandrel.


Schematic illustration of the conventional spinning process (b) Types of parts conventionally spun.All parts are antisymmetric

Shear Spinning

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Metal spinning• Known as power spinning, flow turning, hydro-spinning,

and spin forging• Produces axisymmetric conical or curvilinear shape• Single rollers and two rollers can be used • It has less wastage of material• Typical products are rocket- motor casing and missile nose cones.


Tube spinning :

• Thickness of cylindrical parts are reduced by spinning them on a cylindrical mandrel rollers

• Parts can be spun in either direction• Large tensile elongation up to 2000 % are obtained within

certain temperature ranges and at low strain rates.


Super Plastic forming :Advantages :• Lower strength is required and less tooling costs• Complex shapes with close tolerances can be made • Weight and material savings• Little or no residual stress occurs in the formed parts

Disadvantages :• Materials must not be super elastic at service

temperatures• Longer cycle times


Introduction to Explosive forming• Explosive energy used s metal forming

• Sheet-metal blank is clamped over a die

• Assembly is immersed in a tank with water

• Rapid conversion of explosive charge into gas generates a shock wave .the pressure of this wave is sufficient to form sheet metals


Peak pressure (due to explosion):

caused due to explosion , generated in water

P = k( 3sqrt(w) /R)9

P- in psi

K- constant

TNT- trinitrotoluene

W-weight of explosive in pounds

R- the distance of explosive from the work piece


Diffusion Bonding and Superplastic Forming


Magneticpulse forming

• Suranaree University of Technology Jan-Mar 2007• Stretch formiing• • Forming by using tensile forces• to stretch the material over a tool• or form block.• • used most extensively in the• aircraft industry to produce parts• of large radius of curvature.• (normally for uniform cross• section).


Peen forming• Known as power spinning, flow turning, hydro-spinning,

and spin forging• Produces axisymmetric conical or curvilinear shape• Single rollers and two rollers can be used • It has less wastage of material• Typical products are rocket- motor casing and missile nose cones.


Super plastic forming.Maximum bending force, P = KYLT2

W

K – constant ranges from 0.3(wiping die) – 0.7(u-die)-1.3(V-die)Y – yield stressL- length of the bendT- thickness of sheet

For a V-dieMax bending force, P = (UTS)LT 2

W


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Unit 4 ppt mt1